1. Field of the Invention
The invention relates to a sensing device, and more particularly to a sensing device with back-side illumination which comprises a memory unit disposed on the front side of a sensing pixel array.
2. Description of the Related Art
Conventional image sensors comprises several elements that are key to enhance performance of the image sensors (e.g. CMOS image sensor, CIS), such as color filters, a sensing pixel array which converts light into electrical signals, circuits which receive electrical signals, convert electrical signals to digital signals, and then process the digital signals, and etc. CIS technology is advantageous in that it may be used to integrate all of the above mentioned elements on a single die or chip.
FIG. 1 shows a cross section of a conventional CMOS image sensor with front-side illumination (FSI). Referring to FIG. 1, a CMOS image sensor 1 comprises a pixel array 10 composed of pixels 100, a plurality of metal layers 11 forming CMOS circuits, a color filter 12, and a microlens 13. The metal layers 11 are formed on the pixel array 10 for interconnections. The color filter 12 is formed on the metal layers 11. The CMOS image sensor 1 receives light 14 through the microlens 13, and the received light is transmitted to the pixel array 10 through the color filter 12 and the metal layers 11. Since the CMOS image sensor 1 with front-side illumination requires several light sensing regions which are not blocked by metal lines in the metal layers 11 for a greater aperture ratio, severe restrictions are made at the metal lines connecting the in-pixel circuitry to the peripheral circuitry. These limitations to the metal line interconnection between the pixels 100 and the periphery circuitry limit the maximum bandwidth available for pixel-to-periphery communications. Accordingly, parameters, such as maximum frame rate, dynamic range, etc. of the CMOS image sensor are worsened. Moreover, when the size of the pixels 100 becomes smaller, the performance of the pixel array 10 including quantum efficiency (QE), cross-talk effect, and dark current is degraded.
Thus, back-side illumination (BSI) for CMOS image sensors has been disclosed. FIG. 2 shows a cross section of a conventional CMOS image sensor with back-side illumination. Referring to FIG. 2, a CMOS image sensor 2 comprises a pixel array 20 composed of pixels 200, a plurality of metal layers 21 forming CMOS circuits, a color filter 22, and a microlens 23. The pixel array 20 receives light 24 through the microlens 23 and the color filter 22 and not through metal layers 21. The performance of the pixel array 200 is not hindered by light passing through the metal layers 21, as light passes through the microlens 23 and the color filter 22.